How do I find out who is POA over my brother?

I will take your advice and do some more research.
Thank you all for your feedback.

They are not filed with the court, you will have to ask around and see what you can find out.

Do you have reason to believe your brother ever assigned a POA? I’m not being flippant but many people never assign a POA, make out a living will, etc. Perhaps there is a place in his home where he keeps important paperwork. I would check there. Otherwise, there just might not be any. I hope you find what you need to help your brother.

You would have to ask your brother. POAs generally aren’t filed with the court.

Generally, POAs don't need to be filed with a court. You could try asking other family members. But I don't think there is any legal obligation for this information to be made public, and it might even be considered a privacy violation for the information to be made public without the principal's (i.e., your brother's) permission.